1. Field of the Invention
The present invention relates to a metal oxide electrode for a supercapacitor and a method for manufacturing the same, and more particularly to a metal oxide electrode for a supercapacitor including manganese oxide as an active material of the electrode, and a method for manufacturing the same.
2. Description of the Related Art
The recent advancement of scientific civilization accelerates the use of various high technology electronic devices. These devices are essential for modern life, but such devices produce many environmental problems, such as increasing waste and pollution. Considering those problems, a great deal of effort has been expended to develop an alternative energy storing device having high capacity and long durability without pollution. Also, the need for memory devices which can conveniently control various electronic devices has rapidly increased.
Because of the problem that most electronic devices are subject to memory loss, thus causing errors, when an undesired stoppage or even a variation of power occurs, the need for memory back up power continually increases. To meet such need, much research has been undertaken. One of the best solutions of recent research is development of an electrochemical capacitor, which is called a supercapacitor. It has a greatly enhanced storage capacitance which is more than hundreds to thousands of times larger than that of conventional capacitors. Also, the supercapacitor has high energy density and excellent power density which is hundreds of times more than the power density of a battery, thereby providing much stable and powerful energy to electronic devices.
The electrochemical capacitor is generally divided into three categories, such as an Electric Double Layer Capacitor (EDLC), a metal oxide pseudocapacitor and a conducting polymer capacitor, depending on the energy storage mechanism and active materials used in each system. In the metal oxide pseudocapacitor, the active material is generally conductive metal oxide which has high surface area and electrochemical reactivity with working ions in an electrolyte. Electrochemical reduction and oxidation reactions as well as physical charge separation between an electrode and electrolyte interface, are the energy storage mechanisms. On the other hand, the EDLC uses activated carbon with a large surface area as an active material, and physical charge separation between an electrode and an electrolyte interface is the main energy storage mechanism.
It is generally appreciated that the metal oxide pseudocapacitor can obtain higher capacitance than the EDLC because, as mentioned above, the metal oxide pseudocapacitor can get its capacitance using an electrochemical redox reaction as well as the physical charge separation at the electrolyte interface, whereas the EDLC system only obtains its capacitance from the physical charge separation at the electrolyte interface.
The supercapacitor generally consists of porous active material electrodes, a separator, an electrolyte, a current collector, a case and terminals. The current collector can be composed of high electrical conductivity material, such as metal or a conducting film. The case and the terminals should be composed of light materials to reduce the weight of the capacitor. The separator and the electrolyte relate to the ionic conductivity of the capacitor. The current collector and the terminals are concerned with electrical conductivity of the capacitor. The electrical and the ionic conductivities are the main factors in determining the output characteristics of the capacitor.
The manganese oxide with layered structure can be a candidate for an electrode of the metal oxide pseudocapacitor, which has been studied as an electrode of rechargeable batteries. The manganese oxide, including layered structure having potassium ions therein, is obtained by thermally decomposing potassium permanganate or chemical reactions.
The reaction of the supercapacitor is a surface reaction, while the reaction of the manganese oxide is an intercalation reaction. Thus, such manganese oxide may not be applied to the electrode of the supercapacitor because the supercapacitor has a rapidly charging/discharging, wide permissible temperature range and high electrical conductivity. But, depending on a condition of material synthesis, such as cooling rate, surface condition as a mean valence of manganese ion of material can be changed to show good capacitor performance. Moreover, intercalation reaction also may contribute to the total capacitance of the material in special capacitor operation, such as slow charge and discharge conditions.
Ruthenium oxide (RuO2) has recently been utilized as an electrode of a capacitor. The capacitor using ruthenium oxide as an electrode has a high capacitance of about 700 F/g, which is much higher than that of conventional capacitors. However, ruthenium oxide is too expensive to apply to the capacitor electrodes, i.e., the manufacturing cost of a ruthenium oxide electrode is hundreds of times higher than that of a conventional electrode. Furthermore, the high capacitance of ruthenium oxide can be obtained only when an acid solution, such as that of sulfuric acid (H2SO4), is used therewith, which causes a serious environmental hazard.
The present inventors reported that amorphous manganese oxide has good properties as an electrode for a supercapacitor in a neutral electrolyte, such as potassium chloride (refer to Journal of Solid State Chemistry. vol. 144, pages 220 to 223, 1999, entitled xe2x80x9cSUPERCAPACITOR BEHAVIOR WITH KCI ELECTROLYTExe2x80x9d). However, when amorphous manganese oxide is directly used as the electrode of a supercapacitor, the Equivalent Serial Resistance (ESR) of the supercapacitor may be greatly increased when operating at a high frequency, and the energy loss of the supercapacitor may be seriously increased at a low frequency because amorphous manganese oxide has low conductivity at room temperature.
Although the electrode for the supercapacitor is manufactured by physically mixing conductive carbon having good electrical conductivity with amorphous manganese oxide, the increasing capacitance of the supercapacitor and the reducing volume of the supercapacitor may be limited since little manganese oxide can be included in conductive carbon by specific volume. Also, the physical mixing process has some disadvantages; the contact area between the manganese oxide and the conductive carbon is reduced, and the degree of dispersion of the manganese is limited.
Considering the above-described problems and disadvantages, it is an object of the present invention to provide a metal oxide electrode for a supercapacitor having high capacitance.
It is another object of the present invention to provide a method for manufacturing a metal oxide electrode for a supercapacitor having high capacitance.
It is still another object of the present invention to provide a metal oxide electrode for a supercapacitor having a low ESR and an enhanced high frequency characteristic in a neutral electrolyte.
It is still another object of the present invention to provide a method for manufacturing a metal oxide electrode for a supercapacitor having a low ESR and an enhanced high frequency characteristic in a neutral electrolyte.
To achieve these objects, the present invention provides a metal oxide electrode comprising manganese oxide powder, conductive material and binder.
Preferably, the binder is composed of or comprises polytetrafluoroethylene.
According to one embodiment of the present invention, the conductive material is conductive carbon and the manganese oxide powder is coated on the conductive carbon. As for the present invention, many kinds of highly conductive materials can replace the conductive carbon because the roles of the conductive carbon of the present invention are making an electrical conduction path and sites of the amorphous manganese oxide coating. Therefore, basically all conductive materials, such as metal oxide, metal nitride, metal carbide, metal powder and conducting polymer, are suitable for this purpose.
Preferably, the electrode comprises from approximately 20 to 80% by weight of conductive carbon.
According to the another embodiment of the present, the conductive material is an activated carbon and the manganese oxide powder is coated in pores of the activated carbon and on the surface of the activated carbon.
Preferably, the electrode comprises from approximately 20 to 80% by weight of the activated carbon. In this case, the activated carbon preferably has a specific surface area of from approximately 1500 to 3000 m2/g.
In another embodiment of the present invention, the conductive material is also a conductive carbon and the manganese oxide powder is coated on the conductive carbon. In this case, the electrode preferably includes from approximately 30 to 90% by weight of the manganese oxide powder, from approximately 5 to 50% by weight of the conductive carbon and from approximately 5 to 50% by weight of binder.
Also, to accomplish the above objects of the present invention, there is provided a method for manufacturing a metal oxide electrode for a supercapacitor comprising steps of: forming a conductive material solution by dispersing a conductive material in deionized water; forming a first solution by adding potassium permanganate to the conductive material solution; forming a second solution comprising manganese acetate; forming amorphous manganese oxide by mixing the first solution with the second solution; and forming the metal oxide electrode including the amorphous manganese oxide.
According to one embodiment of the present invention, the step of forming the conductive material solution is performed after dissolving a surfactant in the deionized water. At that time, the surfactant is preferably composed of polyvinylpyrrolidone.
In another embodiment of the present invention, the conductive material is conductive carbon or activated carbon, and the first solution is a potassium permanganate solution, wherein the potassium permanganate is absorbed into the conductive material.
Preferably, the step of forming the metal oxide electrode includes substeps of extracting amorphous manganese oxide powder from the first solution and the second solution, grinding the amorphous manganese oxide powder, forming a mixture by adding binder to the ground amorphous manganese oxide powder and forming an electrode having a predetermined shape by using the mixture.
The step of extracting the amorphous manganese oxide powder preferably includes substeps of filtering the amorphous manganese oxide powder from a mixture of the first solution and the second solution, washing the filtered amorphous manganese oxide powder and drying the washed amorphous manganese oxide powder. In this case, the binder is composed of polytetrafluoroethylene.
According to another embodiment of the present invention, there is provided a method for manufacturing a metal oxide electrode for a supercapacitor comprising steps of: grinding potassium permanganate; heating a furnace at a first predetermined temperature; thermally decomposing the ground potassium permanganate in the furnace; quenching the product to a second predetermined temperature; washing and filtering the product; and mixing the product with conductive material, binder and solution.
Preferably, the furnace is heated to a temperature of from approximately 450 to 550xc2x0 C. and the step of thermally decomposing the ground potassium permanganate is performed to form manganese oxide having a layer structure and comprising potassium ions therein.
The step of quenching the product is performed by rapidly cooling the product to a temperature below room temperature. In this case, the conductive material is conductive carbon and the binder is composed of polytetrafluoroethylene.
In one method for manufacturing a metal oxide electrode for a supercapacitor according to a preferred embodiment of the present invention, a surfactant is sufficiently dissolved in deionized water, and then conductive carbon is dispersed in the deionized water (including the surfactant) to form a conductive carbon solution. After the conductive carbon is sufficiently dispersed in the conductive carbon solution, potassium permanganate (KMnO4) is added to the conductive carbon solution and is absorbed on the surface of the conductive carbon, thereby forming a potassium permanganate solution. Amorphous manganese oxide (MnO2.nH2O) is prepared by mixing the potassium permanganate solution with a manganese acetate solution. Amorphous manganese oxide powder is extracted from the mixed solution through filtering, washing and drying processes. The amorphous manganese oxide powder is ground and mixed with binder to form a mixture. Then, the mixture is formed to manufacture an electrode for a supercapacitor having a predetermined shape. In this case, the electrode comprises from approximately 20 to 80 weight percent (wt %) of the conductive carbon.
As for another method for manufacturing a metal oxide electrode for a supercapacitor according to another preferred embodiment of the present invention, an activated carbon powder is sufficiently dispersed in deionized water to form an activated carbon solution, and then potassium permanganate is added to the activated carbon solution, thereby forming potassium permanganate solution including the potassium permanganate absorbed in pores of the activated carbon and on the surface of the activated carbon. An electrode for a supercapacitor is manufactured according to the above described method after amorphous manganese oxide is formed by mixing the potassium permanganate solution with a manganese acetate solution. In this case, the activated carbon has a specific surface area of from approximately 1500 to 3000m2/g. The electrode comprises from approximately 20 to 80 wt % of the activated carbon. Preferably, the activated carbon has a specific surface area of approximately 2000m2/g, and the electrode is composed of appxomimately 40 wt % of the activated carbon.
In another method for manufacturing a metal oxide electrode for a supercapacitor according to a further preferred embodiment of the present invention, after potassium permanganate is inserted into a furnace previously heated to a predetermined temperature, the potassium permanganate is thermally decomposed in the furnace, and then rapidly cooled below room temperature to form a manganese oxide powder. Such manganese oxide powder has stable chemical structure and composition since the crystal and the particle growths in the manganese oxide powder can be limited during slow heating and slow cooling processes. The thermal decomposition reaction of the potassium permanganate proceeds as the following expression.
10K2MnO4xe2x86x922xc2x765K2MnO4+(2xc2x735K2, 7xc2x735MnO2.O5) +6O2
wherein the subscripts 5 mean significant figures.
The temperature of the furnace is from approximately 450 to 550xc2x0 C. during the thermal decomposition reaction. Preferably, the thermal decomposition reaction proceeds at temperature of approximately 500xc2x0 C.
If the thermal decomposition reaction occurs below 450xc2x0 C., manganese oxide cannot be obtained since the thermal decomposition reaction begins only above 450xc2x0 C. Also, surface conditions of the manganese oxide, including mean valence of manganese ion, are not adequate to yield good capacitance when the thermal decomposition reaction occurs above 650xc2x0 C.
FIG. 1 is a graph showing X-ray diffraction analysis of the manganese oxide powder according to a preferred embodiment of the present invention. Such manganese oxide is manufactured by washing the thermally decomposed potassium permanganate with distilled water and drying it at 120xc2x0 C. after the potassium permanganate is thermally treated at approximately 500xc2x0 C. for approximately 2 hours and rapidly cooled to room temperature.
Referring to FIG. 1, the manganese oxide powder of the present invention has nearly a delta (xcex4) phase as the layered structure.
FIG. 2 is a graph showing particle size distribution of the manganese oxide powder in FIG. 1. As shown in FIG. 2, most of the particles of the manganese oxide powder have diameters of from approximately 0.1 to 1.0 xcexcm.
FIG. 3 is a graph illustrating measured capacitances of manganese oxide powders respectively prepared at various temperatures according to another preferred embodiments, and FIG. 4 is a graph illustrating unit capacitances of the manganese oxide powders respectively manufactured for various reaction times according to another preferred embodiments. In FIG. 4, the thermal decomposition reactions occur at approximately 500xc2x0 C.
Referring to FIGS. 3 and 4, the manganese oxide powder manufactured at 500xc2x0 C. for 2 hours has the highest specific capacitance.
When the manganese oxide having layered structure is applied to the electrode for the supercapacitor, the conductive material, such as the conductive carbon, is mixed with the manganese oxide since the manganese oxide having the layered structure has relatively low electrical conductivity. Otherwise, the energy of the capacitor may reduce when the capacitor is operated at low frequency. Also, the binder is added to the conductive material having the manganese oxide coated thereon for coating the mixture on a current collector to have a film shape. If the electrode comprises below 30 wt % of the manganese oxide or above 80 wt % of the conductive material, the electrode cannot have proper characteristics for the supercapacitor. When the electrode comprises above 90 wt % of the manganese oxide or below 5 wt % of the conductive material, the ESR of the electrode increases, so the capacitor having such an electrode may operate only with difficulty at a high frequency or at a low frequency. In addition, the mixture cannot be coated on the current collector when the electrode comprises below 5 wt % of the binder and the electrode may not be applied to the supercapacitor when the electrode includes above 50 wt % of the binder. The electrode of the present invention has a high capacitance of approximately 300 F/g in case of a water-soluble electrolyte.
According to the present invention, the electrode for the supercapacitor can reduce the ESR and enhance high frequency characteristics since the contact area and the adhesion strength between the manganese oxide and the conductive carbon are improved.
Also, the electrode of the present invention has high capacitance suitable for a supercapacitor as compared with conventional electrodes because the electrode is manufactured by mixing amorphous manganese oxide powder with the conductive material and the binder.
Furthermore, the electrode of the present invention can be manufactured at a cost of one percent of that of the ruthenium oxide electrode, even though the electrode of the present invention has good capacitance nearly equal to half the capacitance of the ruthenium oxide electrode which has the highest capacitance among conventional electrodes.